Method for Precoding Matrix Indicator Feedback and Apparatus

ABSTRACT

A method and apparatus for precoding matrix indicator feedback reduce a quantity of bits in the feedback. In the method, a terminal device receive a reference signal from a base station, determines a precoding matrix W in a precoding matrix set corresponding to a rank indication. W satisfies W=W 1 ×W 2 ×W 3 , W 1 , W 2  and W 3  and matrices and respectively corresponding to a first, second and third precoding matrix indicator. None of W 1 , W 2 , and W 3  is an identity matrix, and the 2M columns in W 1  comprise every column in W 1 ×W 2 . The terminal device transmits the rank indication, the first precoding matrix indicator, the second precoding matrix indicator, and the third precoding matrix indicator to the base station. Therefore, a quantity of to-be-selected vectors is reduced by using a first-stage feedback and a second-stage feedback, thereby reducing calculation complexity of third-stage feedback, and reducing a quantity of bits in the third-stage feedback.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/248,344, filed on Jan. 15, 2019, which is a continuation of U.S.patent application Ser. No. 16/148,296, filed on Oct. 1, 2018, now U.S.Pat. No. 10,419,090, which is a continuation of InternationalApplication No. PCT/CN2016/078298, filed on Apr. 1, 2016. All of theafore-mentioned patent applications are hereby incorporated by referencein their entireties.

TECHNICAL FIELD

This application relates to the mobile communications field, and inparticular, to a multiple-antenna technology in a wirelesscommunications system.

BACKGROUND

Multiple-input and multiple-output (MIMO) technologies are widelyapplied in a Long Term Evolution (LTE) system. A transmitting end mayuse a precoding technology to process data, so as to improve signaltransmission quality or a signal transmission rate. The transmitting endmay be a base station or a terminal device.

In the LTE system, a base station obtains a preceding matrix usually ina manner in which a terminal device feeds back a preceding matrixindicator (PMI). A set of used preceding matrices is usually referred toas a codebook or a codebook set. Each preceding matrix in the codebookor the codebook set may also be referred to as a codeword.

The terminal device quantizes channel state information (CSI) and feedsback the CSI to the base station. The base station determines apreceding matrix based on the CSI. CSI information fed back in anexisting LTE system includes a rank indication (RI), a PMI, and thelike. The RI is used to indicate a quantity of data streams for spatialmultiplexing. The PMI is used to indicate a precoding matrix.

An LTE standard supports 8 antenna ports and 16 antenna ports.Currently, a dual-stage codebook feedback mechanism is defined to reducea quantity of bits of a PMI fed back by a terminal device, so as toreduce load. First-stage feedback indicates a vector group, includingfour vectors for subsequent processing. One of the four vectors isselected for second-stage feedback, and the selected vector may be usedfor data precoding. The first-stage feedback has a long period/a wideband characteristics, and the second-stage feedback has a short period/asub-band characteristics.

FIG. 1 is a schematic diagram of a two-dimensional antenna array. InFIG. 1, an antenna port has 45 degrees polarization and −45 degreespolarization. For a polarization direction, there are N₁ antenna portsin a horizontal direction, and there are N₂ antenna ports in a verticaldirection.

As a quantity of antenna ports increases, a beam width obtained afterthe base station performs precoding becomes increasingly narrow. Tobetter cover an entire system bandwidth, the vector group in thefirst-stage feedback needs to include more vectors. For example, for 32antenna ports, 16 antenna ports are included in each polarizationdirection. To enable a vector group in first-stage feedback in a systemwith 32 antenna ports and a vector group in first-stage feedback in asystem with 16 antenna ports to have same coverage space, the vectorgroup in the first-stage feedback in the system with the 32 antennaports needs to include 16 column vectors, as shown in FIG. 2a and FIG. 2b.

In a case of the 32 antenna ports, if the current codebook feedbackmechanism is still used, to be specific, the vector group in thefirst-stage feedback includes four vectors, a coverage bandwidth isinevitably affected, leading to performance degradation of a codebook.

If a quantity of vectors in the first-stage feedback is increased to 16,the 16 vectors need to be used for selection for the second-stagefeedback. This increases a quantity of bits in the second-stagefeedback, and consequently, increases system overheads of the terminaldevice.

SUMMARY

This application describes a precoding matrix indicator feedback methodand an apparatus, so as to reduce system overheads of a terminal devicewhile satisfying a system performance requirement as much as possible.

According to a first aspect, a precoding matrix determining method isprovided. A terminal device determines a preceding matrix W in apreceding matrix set corresponding to a rank indication. W satisfiesW=W₁×W₂×W₃, W is a matrix of N_(t) rows and R columns, N_(t) is aquantity of antenna ports, R is a rank value corresponding to the rankindication, N_(t) is greater than or equal to R. W₁ is a matrix of N_(t)rows and 2M columns, W₂ is a matrix of 2M rows and 2K columns, W₃ is amatrix of 2K rows and R columns, K is less than M. N_(t), R, M, and Kare all positive integers, M is greater than or equal to 2, N_(t) is aneven number, none of W₁, W₂, and W₃ is an identity matrix, and the 2Mcolumns in W₁ include every column in W₁×W₂. A first preceding matrixindicator corresponds to the first precoding matrix W₁, a secondprecoding matrix indicator corresponds to the second precoding matrixW₂, and a third precoding matrix indicator corresponds to the thirdprecoding matrix W₃. The terminal device transmits the rank indication,the first precoding matrix indicator, the second precoding matrixindicator, and the third precoding matrix indicator to the base station.

A set of columns in W₁ include every column in W₁×W₂. This representsthat 2K column vectors are selected from the columns in W₁ as a resultof W₁×W₂. In this way, a quantity of vectors in a set of to-be-selectedvectors is reduced subsequently, thereby reducing calculation complexityof subsequent processing, and reducing a quantity of bits for feedingback a PMI for selection from candidate vectors.

According to a second aspect, a precoding matrix indicator feedbackmethod is provided. A base station receives a rank indication, a firstprecoding matrix indicator, a second precoding matrix indicator, and athird precoding matrix indicator a terminal device. The base stationdetermines in a precoding matrix set corresponding to the rankindication, a precoding matrix W based on the first precoding matrixindicator, the second precoding matrix indicator, and the thirdprecoding matrix indicator. W satisfies W=W₁×W₂×W₃, W is a matrix ofN_(t) rows and R columns. N_(t) is a quantity of antenna ports, R is arank value corresponding to the rank indication, N_(t) is greater thanor equal to R. W₁ is a matrix of N_(t) rows and 2M columns, W₂ is amatrix of 2M rows and 2K columns, W₃ is a matrix of 2K rows and Rcolumns, K is less than M, N_(t), R, M, and K are all positive integers,M is greater than or equal to 2. N_(t) is an even number, none of W₁,W₂, and W₃ is an identity matrix, and the 2M columns in W₁ include everycolumn in W₁×W₂.

The first precoding matrix indicator corresponds to the first precodingmatrix W₁, the second precoding matrix indicator corresponds to thesecond precoding matrix W₂, and the third precoding matrix indicatorcorresponds to the third precoding matrix W₃.

According to a third aspect, an embodiment of the present inventionprovides a terminal device. The terminal device has a function ofimplementing behaviors of the terminal device in the foregoing methoddesigns. The function may be implemented by hardware, or may beimplemented by hardware executing corresponding software. The hardwareor the software includes one or more modules corresponding to theforegoing functions. The modules may be software and/or hardware.

The terminal device includes a receiver, a processor and a transmitter.The receiver is configured to receive a reference signal from a basestation. The processor is configured to determine a precoding matrix Win a precoding matrix set corresponding to a rank indication. Wsatisfies W=W₁×W₂×W₃, W is a matrix of N_(t) rows and R columns, N_(t)is a quantity of antenna ports, R is a rank value corresponding to therank indication. N_(t) is greater than or equal to R. W₁ is a matrix ofN_(t) rows and 2M columns, W₂ is a matrix of 2M rows and 2K columns, W₃is a matrix of 2K rows and R columns, K is less than M. N_(t), R, M, andK are all positive integers, M is greater than or equal to 2, N_(t) isan even number. None of W₁, W₂, and W₃ is an identity matrix, and the 2Mcolumns in W₁ include every column in W₁×W₂. A first precoding matrixindicator corresponds to the first precoding matrix W₁, a secondprecoding matrix indicator corresponds to the second precoding matrixW₂, a third precoding matrix indicator corresponds to the thirdprecoding matrix W₃.

The transmitter is configured to send the rank indication, the firstprecoding matrix indicator, the second precoding matrix indicator, andthe third precoding matrix indicator to the base station.

Optionally, receiver is configured to receive a configuration parametersent by the base station.

According to a fourth aspect, an embodiment of the present inventionprovides a base station. The base station has a function of implementingbehaviors of the base station in the foregoing method designs. Thefunction may be implemented by hardware, or may be implemented byhardware executing corresponding software. The hardware or the softwareincludes one or more modules corresponding to the foregoing functions.

The base station includes a receiver and a processor. The receiver isconfigured to receive a rank indication, a first precoding matrixindicator, a second precoding matrix indicator, and a third precodingmatrix indicator from a terminal device. The processor is configured todetermine in a precoding matrix set corresponding to the rank indicationa precoding matrix W based on the first precoding matrix indicator, thesecond precoding matrix indicator, and the third precoding matrixindicator. W satisfies W=W₁×W₂×W₃, W is a matrix of N_(t) rows and Rcolumns, N_(t) is a quantity of antenna ports, R is a rank valuecorresponding to the rank indication, N_(t) is greater than or equal toR. W₁ is a matrix of N_(t) rows and 2M columns, W₂ is a matrix of 2Mrows and 2K columns, W₃ is a matrix of 2K rows and R columns, K is lessthan M. N_(t), R, M, and K are all positive integers, M is greater thanor equal to 2, N_(t) is an even number. None of W₁, W₂, and W₃ is anidentity matrix, and the 2M columns in W₁ include every column in W₁×W₂.The first precoding matrix indicator corresponds to the first precodingmatrix W₁, the second precoding matrix indicator corresponds to thesecond precoding matrix W₂, and the third precoding matrix indicatorcorresponds to the third precoding matrix W₃.

Optionally, the base station further includes a transmitter configuredto send a configuration parameter.

In the first to the fourth aspects, further, there may be the followingoptional designs:

Optionally, each precoding matrix W in the precoding matrix setcorresponding to the rank indication satisfies W=W₁×W₂×W₃.

Optionally, W₂ satisfies

${W_{2} = \begin{bmatrix}X_{2} & 0 \\0 & X_{2}\end{bmatrix}},$

X₂ is a matrix of M rows and K columns, any column in X₂ is representedas e_(p), e_(p) is an M×1 column vector, a p^(th) element in e_(p) is 1,remaining elements are 0, and p is an integer from 1 to M.

Optionally, W₁ satisfies

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$

X₁ is a matrix of N_(t)/2 rows and M columns, X₁=[v₀ . . . v_(M-1)], v₀is a column vector including N_(t)/2 elements, and o is an integer from0 to M−1; and

Any column in W₃ is represented as

$\begin{bmatrix}e_{l} \\{\varphi_{n}e_{l}}\end{bmatrix},$

ϕ_(n) is a complex number, e_(l) is a K×1 column vector, an l^(th)element in e_(l) is 1, remaining elements are 0, and l is an integerfrom 1 to K.

Optionally, a frequency domain resource corresponding to the firstprecoding matrix indicator is a downlink system bandwidth of theterminal device. The downlink system bandwidth includes A firstsub-bands and B second sub-bands, A and B are positive integers greaterthan 1, and A is less than or equal to B.

A frequency domain resource corresponding to the second precoding matrixindicator is one of the A first sub-bands, and a frequency domainresource corresponding to the third precoding matrix indicator is one ofthe B second sub-bands.

A quantity of column vectors in a vector group corresponding to thefirst sub-band is less than a quantity of vectors in a vector groupcorresponding to the system bandwidth. Therefore, a quantity of vectorsthat need to be searched during vector selection on the second sub-bandis reduced, and a quantity of bits required for feeding back anindicator of the selected vector on the second sub-band can be reduced.

Optionally, a frequency domain resource of at least one of the A firstsub-bands is the same as frequency domain resources of at least two ofthe B second sub-bands.

Optionally, frequency domain resources corresponding to the firstprecoding matrix indicator, the second precoding matrix indicator, andthe third precoding matrix indicator are downlink system bandwidths ofthe terminal device.

Optionally, a transmission period of the first precoding matrixindicator is P₁, a transmission period of the second precoding matrixindicator is P₂, a transmission period of the third precoding matrixindicator is P₃, P₁ is greater than or equal to P₂, and P₂ is greaterthan or equal to P₃.

Optionally, the transmission periods P₁, P₂, and P₃ are sent by the basestation to the terminal device through Radio Resource Control (RRC)signaling.

Different transmission periods are configured for different precodingmatrix indicators, and are used to correspond to different features of achannel. Some precoding matrix indicators correspond to a part that isof the channel and that varies relatively fast over time, and someprecoding matrix indicators correspond to a part that is of the channeland that varies relatively slowly over time. For example, the firstprecoding matrix indicator corresponds to a part that is of a channeland that varies most slowly over time, the second precoding matrixindicator corresponds to a part that is of the channel and that variesrelatively slowly over time, and the third precoding matrix indicatorcorresponds to a part that is of the channel and that varies relativelyfast over time. P₁, P₂, and P₃ are configured based on channel features,so as to reduce a quantity of bits for feeding back a PMI.

Optionally, T column vectors in 2K column vectors in W₂ are indicated bythe first precoding matrix indicator, T is an integer greater than orequal to 2, and T is less than K; and 2K−T column vectors in W₂ exceptthe T column vectors are indicated by the T column vectors and thesecond precoding matrix indicator. In this way, a quantity of bitsrequired for feeding back the second _(p) recoding matrix indicator isreduced.

Optionally, 2K column vectors in W₂ are indicated by a configurationparameter delivered by a base station and the second precoding matrixindicator. In this way, a quantity of bits required for feeding back thesecond precoding matrix indicator is reduced.

Optionally, the configuration parameter is used to indicate a selectablecolumn vector set of W₁, the selectable column vector set includes Jcolumn vectors, and J satisfies 2K<J<2M.

Optionally, the configuration parameter is a configuration parametersent by the base station to the terminal device through RRC signaling.

Optionally, X₁ in W₁ satisfies X₁=[v₀ ¹ . . . v_(M) ₁ ₋₁ ¹]⊗[v₀ ² . . .v_(M) ₂ ₋₁ ²], where

v_(m) ¹ is a column vector including N₁ elements, v_(n) ² is a columnvector including N₂ elements, N₁×N₂=N_(t)/2, M₁×M₂=M, and ⊗ represents aKronecker product.

Optionally, X₂ in W₂ satisfies X₂=X₃⊗X₄, where X₃ is a matrix of M₁ rowsand K₁ columns, X₄ is a matrix of M₂ rows and K₂ columns, and ⊗represents a Kronecker product.

Any column in X₃ is represented as e_(i), e_(i) is an M₁×1 columnvector, an i^(th) element in e_(i) is 1, remaining elements are 0, and avalue of i is an integer from 1 to M₁.

any column in X₄ is represented as e_(j), e_(j) is an M₂×1 columnvector, a j^(th) element in e_(j) is 1, remaining elements are 0, and avalue of j is an integer from 1 to M₂.

K ₁ ×K ₂ =K, and M ₁ ×M ₂ =M.

In the third and the fourth aspects, the sending unit may be atransmitter, the receiving unit may be a receiver, and the processingunit may be a processor.

An embodiment of the present invention further provides a system. Thesystem includes the terminal device and the base station in theforegoing embodiments.

In this application, feedback (or sending) of the first precoding matrixindicator may be considered as first-stage feedback, feedback of thesecond precoding matrix indicator may be considered as second-stagefeedback, and feedback of the third preceding matrix indicator may beconsidered as third-stage feedback.

Compared with the prior art, in the solutions provided in thisapplication, feedback of a precoding matrix indicator is classified intothree stages. The second-stage feedback is used to indicate some vectorsin a vector group indicated by the first-stage feedback. Due to areduction in a quantity of to-be-selected vectors, system overheads ofcalculating the third-stage feedback by the terminal are reduced, and aquantity of bits required for the third-stage feedback is reduced,thereby better balancing system performance and feedback overheads ofthe terminal device.

BRIEF DESCRIPTION OF DRAWINGS

To describe the technical solutions in the embodiments of the presentinvention more clearly, the following briefly describes the accompanyingdrawings for describing the embodiments.

FIG. 1 is a schematic diagram of a two-dimensional antenna array:

FIG. 2a is a schematic diagram of coverage space of a beam group in acase of 8 antenna ports:

FIG. 2b is a schematic diagram of coverage space of a beam group in acase of 32 antenna ports;

FIG. 3 is a schematic flowchart of a preceding matrix indicator feedbackmethod according to an embodiment of the present invention;

FIG. 4 is a schematic diagram of a correspondence between a multipathand a vector group;

FIG. 5 is a schematic diagram of a first sub-band and a second sub-band;

FIG. 6 is a schematic block diagram of a terminal device according to anembodiment of the present invention;

FIG. 7 is a schematic block diagram of a base station according to anembodiment of the present invention;

FIG. 8 is a schematic block diagram of a terminal device according toanother embodiment of the present invention; and

FIG. 9 is a schematic block diagram of a base station according toanother embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

The technical solutions according to embodiments of the presentinvention are clearly described in the following with reference to theaccompanying drawings. Apparently, the described embodiments are merelysome but not all of the embodiments of the present invention. All otherembodiments obtained by a person of ordinary skill in the art based onthe embodiments of the present invention without creative efforts shallfall within the protection scope of the present invention.

Network architectures and business scenarios described in theembodiments of the present invention aim to more clearly describe thetechnical solutions in the embodiments of the present invention, but arenot intended to limit the technical solutions provided in theembodiments of the present invention. A person of ordinary skill in theart may know that as the network architectures evolve and a new businessscenario emerges, the technical solutions provided in the embodiments ofthe present invention further apply to a similar technical problem.

It should be understood that the technical solutions in the embodimentsof the present invention may be applied to various communicationssystems, for example, a Long Term Evolution (LTE) system, an LTEfrequency division duplex (FDD) system, an LTE time division duplex(TDD) communications system, a device to device (D2D) communication, orthe like.

It should also be understood that in the embodiments of the presentinvention, the terminal device (terminal equipment) may be referred toas a terminal, or may be user equipment (UE), a mobile station (MS), amobile terminal, or the like. The terminal device may communicate withone or more core networks by using a radio access network (RAN). Forexample, the terminal device may be a mobile phone (or referred to as a“cellular” phone) or a computer with a mobile terminal. For example, theterminal device may also be a portable, pocket-sized, handheld, computerbuilt-in, or in-vehicle mobile apparatus, which exchanges voice and/ordata with the radio access network.

In the embodiments of the present invention, the base station may be anevolved base station (evolved node B, “eNB or e-NodeB” for short) inLTE, or may be another base station, or an access network device such asa relay. For the D2D communication, the base station may also be adevice in the D2D communication. This is not limited in the presentinvention. For convenience of description, the following embodiments aredescribed by using the eNB as an example.

FIG. 3 is a schematic flowchart of a precoding matrix determining methodaccording to an embodiment of the present invention. As shown in FIG. 3,the method includes the following steps.

Step 101: A terminal device determines a rank indication.

In step 101, one rank value corresponds to one rank indication. Theterminal device sends the rank indication to a base station, to indicatea quantity of downlink data streams that the terminal device expects touse for spatial multiplexing. For example, the rank value ranges from 1to 8, and the rank indication is represented by three bits. When therank indication is 000, it represents that a rank is 1, when the rankindication is 001, it represents that a rank is 2, and by analog. Inconclusion, when a value is taken for a rank, there is a value of a rankindication corresponding to the rank.

Optionally, the terminal device may determine, based on information suchas channel state information and the like, a quantity of data streamsfor spatial multiplexing, namely, a rank. Optionally, the base stationsends a cell-specific reference signal (CRS) or a channel stateinformation reference signal (CSI-RS) to the terminal device. Theterminal device obtains downlink channel estimation and downlinkinterference estimation based on the CRS or the CSI-RS, and thendetermines, based on the downlink channel estimation and the downlinkinterference estimation, a quantity of transmitted downlink data streamsthat the terminal device expects to use for spatial multiplexing duringdownlink transmission, namely, a rank. It should be understood that theterminal device may determine the rank by using a method well-known to aperson skilled in the art. For brevity, details are not describedherein.

Step 102: Determine a precoding matrix in a precoding matrix setcorresponding to the rank indication.

In step 102, the terminal device may determine, in a precoding matrixset corresponding to the rank indication and based on a reference signalsuch as the CSI-RS, a precoding matrix that the terminal device expectsthe base station eNB to use when the base station sends downlink data.

Step 103: Determine a first precoding matrix indicator, a secondprecoding matrix indicator, and a third precoding matrix indicator thatare used to indicate the precoding matrix.

For the base station and the terminal device, there is a precodingmatrix set for each rank (or each rank indication). In addition, in acase of a given rank, each precoding matrix is indicated by a firstprecoding matrix indicator, a second precoding matrix indicator, and athird precoding matrix indicator. There is a correspondence between aprecoding matrix W and a first precoding matrix indicator, a secondprecoding matrix indicator, and a third precoding matrix indicator. Forexample, for a rank 1, the first precoding matrix indicator, the secondprecoding matrix indicator, and the third precoding matrix indicatoreach are represented by two bits, and there are a total of six bits torepresent the three precoding matrix indicators. A precoding matrix setfor the rank 1 includes 2{circumflex over ( )}6=64 precoding matrices.Each precoding matrix W corresponds to three precoding matrixindicators. For example, a value of the first precoding matrix indicatoris 00, a value of the second precoding matrix indicator is 00, and avalue of the third precoding matrix indicator is 00. This is equivalentto that a 6-bit precoding matrix indicator is 000000, and corresponds toone precoding matrix W. When the 6-bit precoding matrix indicator is000001, the 6-bit precoding matrix indicator corresponds to anotherprecoding matrix W. By analog, each precoding matrix W has a one-to-onecorrespondence with a precoding matrix indicator. This is equivalent tothat when a value of a precoding matrix indicator is known, acorresponding precoding matrix is determined.

Step 104: The terminal device sends the rank indication, the firstprecoding matrix indicator, the second precoding matrix indicator, andthe third precoding matrix indicator to a base station.

For example, on a physical uplink shared channel (PUSCH) or anotherchannel, the terminal device sends, to the base station, the rankindication and a precoding matrix indicator (the first precoding matrixindicator, the second precoding matrix indicator, and the thirdprecoding matrix indicator) that is used to indicate the precodingmatrix. The base station may configure the terminal device to feed backthe preceding matrix indicator on the PUSCH, or to feed back theprecoding matrix indicator on a physical uplink control channel (PUCCH).For sending of the rank indication, the first preceding matrixindicator, the second precoding matrix indicator, and the thirdprecoding matrix indicator by the terminal device, the rank indicationmay be sent before the three precoding matrix indicators are sent, ormay be sent together with a part of the three precoding indicators, ormay be sent together with all of the three precoding indicators. Thereis no sequence limit on sending of the rank indication and the precedingmatrix indicators.

On a side of the base station, in step 105, the base station receivesthe rank indication and the precoding matrix indicators sent by theterminal device. For receiving of the rank indication, the firstprecoding matrix indicator, the second precoding matrix indicator, andthe third preceding matrix indicator by the base station, the rankindication may be received before the three precoding matrix indicatorsare received, or may be received together with a part of the threepreceding indicators, or may be received together with all of the threeprecoding indicators. There is no sequence limit on receiving of therank indication and the precoding matrix indicators.

On the side of the base station, in step 106, in a codebook setcorresponding to the rank indication, the precoding matrix is determinedbased on the first precoding matrix indicator, the second precodingmatrix indicator, and the third precoding matrix indicator.

On the side of the base station, in step 107, the base station sendsdata based on the precoding matrix.

Optionally, in step 107, the base station sends data to the terminaldevice based on the precoding matrix determined in step 106. The basestation may send the data to the terminal device on a physical downlinkshared channel (PDSCH). When the data is sent, the precoding matrix usedby the base station may be a precoding matrix corresponding to theprecoding matrix indicators fed back by the terminal device, or anotherprecoding matrix obtained after this precoding matrix is transformed,for example, considering a zero-forcing algorithm at a transmitting endin multiuser MIMO.

W satisfies W=W₁×W₂×W₃, W is a matrix of N_(t) rows and R columns, N_(t)is a quantity of antenna ports, R is a rank value corresponding to therank indication, N_(t) is greater than or equal to R, W₁ is a matrix ofN_(t) rows and 2M columns, W₂ is a matrix of 2M rows and 2K columns, W₃is a matrix of 2K rows and R columns. K is less than M, N_(t), R, M, andK are all positive integers, M is greater than or equal to 2, N_(t) isan even number, none of W₁, W₂, and W₃ is an identity matrix, and the 2Mcolumns in W₁ include every column in W₁×W₂.

The precoding matrix W corresponds to a first precoding matrixindicator, a second precoding matrix indicator, and a third precodingmatrix indicator, the first precoding matrix indicator corresponds tothe first precoding matrix W₁, the second precoding matrix indicatorcorresponds to the second precoding matrix W₂, and the third precodingmatrix indicator corresponds to the third preceding matrix W₃.

A set of columns in W₁ include every column in W₁×W₂. This representsthat 2K column vectors are selected from the columns in W₁ as a resultof W₁×W₂. In this way, a quantity of vectors in a set of to-be-selectedvectors is reduced subsequently, thereby reducing calculation complexityof subsequent processing, and reducing a quantity of bits for feedingback a PMI for selection from candidate vectors. For example, W₂ is usedto select a column vector from W₁, so that a quantity of selectablevector sets in W₃ is reduced, thereby reducing a quantity of bitsrequired for feeding back W₃, and reducing calculation complexity.

In this embodiment of the present invention, in an optionalimplementation, each precoding matrix W in the precoding matrix setcorresponding to the rank indication satisfies W=W₁×W₂×W₃.

As a quantity of antenna ports in a horizontal direction or a verticaldirection increases, a formed beam becomes increasingly thinner, and itis even possible that one beam can cover only one path in a multipathchannel. To capture more channel paths, that the terminal device selectsand feeds back a plurality of vector groups for the base station toperform preceding is a good method. The vector group may include four orless vectors, for example, including two vectors. This method isdescribed below by using an example in which there are 32 antenna portswith an antenna configuration (N₁, N₂)=(16, 1) (as shown in FIG. 4). N₁is a quantity of antenna ports in a polarization direction (45 degreespolarization or −45 degrees polarization) in a first direction, and N₂is a quantity of antenna ports in a polarization direction in a seconddirection, where N₁×N₂=N_(t)/2. The first direction may be a horizontaldirection (or a vertical direction), and the second direction may be avertical direction (or a horizontal direction).

In such an antenna form, an entire system bandwidth can be covered onlywhen W₁ includes 16 vectors. For example,

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$

where W₁ is a block diagonal matrix, and two diagonal matrices are thesame in W₁.

A quantity of vectors included in W₁ is a quantity of column vectorsincluded in X₁, and is not a quantity of column vectors in W₁. Aquantity of column vectors in W₁ is twice that of column vectorsincluded in X₁. Usually, when a column vector in X₁ acts on an antennaport in a polarization direction, a beamforming function can be played.For example, each column in X₁ is a DFT (DFT, Discrete FourierTransform, discrete Fourier transform) vector. Therefore, each columnvector in X₁ may be considered as a direction vector or a beam vector.

The DFT vector refers to a T×1 precoding matrix, and the DFT vector vusually has a form shown by the following equation:

v=[1 e ^(j2πm/N) . . . e ^(j2π(T-2)m/N) e ^(j2π(T-1)m/N)]^(T)  (1)

where N and m are integers (N is not equal to 0), usually, N=2^(x), x isa nonnegative integer, in other words, N is x powers of 2, and a t^(th)element in the DFT vector v is e^(j2π(t-1)m/N) (t=1, 2, . . . , T). Asuperscript ^(T) represents matrix transposing. One diagonal block X₁ inW₁ is used for one polarization direction, and another diagonal block X₁is used for another polarization direction. In an example of FIG. 4,antenna ports are arranged in a horizontal direction, and there are 16antenna ports in one polarization direction. In this case, a quantity ofrows in a matrix X₁ is 16. W₁ includes 16 vectors. This is equivalent tothat X₁ has 16 columns. For example, each column in X₁ may be a DFTvector. Every four vectors form one vector group in X₁, and there are atotal of four vector groups. For example, first to fourth columns in X₁are used as a first vector group, fifth to eighth columns in X₁ are usedas a second vector group, ninth to twelfth columns in X₁ are used as athird vector group, and thirteenth to sixteenth columns in X₁ are usedas a fourth vector group.

The 16 vectors in W₁ include a vector that the terminal device expects,within a relatively long time, the base station to use during precoding.However, actually, at a moment, not each vector in the vector group canbe used. For example, for a terminal device, there are two strong pathsfrom the terminal device to an antenna port of the base station. For thefirst strong path, when the base station uses a vector in the firstvector group to perform preceding, a generated beam can aim to the firststrong path in a direction of a main lobe. When the base station uses avector in the fourth vector group to perform precoding, a generated beamcan aim to the second strong path in the direction of the main lobe, asshown in FIG. 4.

In this case, the terminal device needs to send the first precodingmatrix indicator (including 16 beam vectors) corresponding to W₁, andfurther feed back the second preceding matrix indicator of one or morevector groups selected from W₁. For example, in this example, theterminal device sends indicators of the first vector group and thefourth vector group to the base station.

When the terminal device learns, through measurement, that a channelrank is greater than 2, the terminal device needs to select, for eachreported vector group, one or more vector groups orthogonal to thereported vector group, and send numbers of the one or more selectedorthogonal vector groups to the base station. For example, a rankdetermined by the terminal device is 4, and vector groups reported bythe terminal device are numbered as 1 and 4. Vectors included in thevector group numbered 1 are [v₁ v₂ v₃ v₄], and vectors included in thevector group numbered 2 are [v₅ v₆ v₇ v₈]. In addition, the terminalfurther needs to send, to the base station, a vector group 1+korthogonal to the vector group 1 and a vector group 4+k′ orthogonal tothe vector group 4, where vectors in the vector group numbered 1+k are[v_(1+k) v_(2+k) v_(3+k) v_(4+k)], and vectors in the vector groupnumbered 1+k′ are [v_(5+k′) v_(6+k′) v_(7+k′) v_(8+k′)]. Based on theforegoing descriptions,

${W_{1} \times W_{2}} = \begin{bmatrix}Y & 0 \\0 & Y\end{bmatrix}$

and Y=[v₁ . . . v₈ v_(1+k) . . . v_(4+k) v_(5+k′) . . . v_(8+k′)] areobtained.

In this embodiment of the present invention, in an optionalimplementation, W₂ satisfies

${W_{2} = \begin{bmatrix}X_{2} & 0 \\0 & X_{2}\end{bmatrix}},$

X₂ is a matrix of M rows and K columns, any column in X₂ is representedas e_(p), e_(p) is an M×1 column vector, a p^(th) element in

$W_{2} = \begin{bmatrix}X_{2} & 0 \\0 & X_{2}\end{bmatrix}$

is 1, remaining elements are 0, and p is an integer from 1 to M.

In this embodiment of the present invention, in an optionalimplementation, W₁ satisfies

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$

X₁ is a matrix of N_(t)/2 rows and M columns, X₁=[v₀ . . . v_(M-1)], v₀is a column vector including N_(t)/2 elements, and o is an integer from0 to M−1.

Any column in W₃ is represented as

$\begin{bmatrix}e_{l} \\{\varphi_{n}e_{l}}\end{bmatrix},$

ϕ_(n) is a complex number, l_(i) is a K×1 column vector, an l^(th)element in l_(l) is 1, remaining elements are 0, and l is an integerfrom 1 to K.

For example, W₁ may be represented as

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$

X₁=[v_(k) ¹ . . . v_(k+L) ₁ ⁻¹ ¹]⊗[v_(l) ² . . . v_(l+L) ₂ ⁻¹ ²], where

$v_{k + m}^{1} = \begin{bmatrix}1 & e^{j\; \frac{2{\pi {({k + m})}}}{N_{1}O_{1}}} & \ldots & e^{j\; \frac{2{\pi {({N_{1} - 1})}}{({k + m})}}{N_{1}O_{1}}}\end{bmatrix}^{T}$

represents an m precoding vector in the first direction, m=0, 1, . . . ,L₁−1, L₁ represents a quantity of column vectors in the first directionin X₁ in W₁,

$v_{l + n}^{2} = \begin{bmatrix}1 & e^{j\; \frac{2{\pi {({l + n})}}}{N_{2}O_{2}}} & \ldots & e^{j\frac{\; {2{\pi {({N_{2} - 1})}}{({l + n})}}}{N_{2}O_{2}}}\end{bmatrix}^{T}$

represents an n^(th) precoding vector in the second direction, n=0, 1, .. . , L₂−1, L₂ represents a quantity of column vectors in the seconddirection in X₁ in W₁, and both Q₁ and Q₂ are generation parameters forgenerating the foregoing column vectors, and are positive integers. Thefirst direction may be a horizontal direction (or a vertical direction),and the second direction may be a vertical direction (or a horizontaldirection).

W₂ is used to further select K column vectors from vector groups in W₁,and may be represented as

${W_{2} = \begin{bmatrix}X_{2} & 0 \\0 & X_{2}\end{bmatrix}},$

where X₂=[e_(i) . . . e_(j)]_(M×K), X₂ is a matrix of M rows and Kcolumns, l_(i) represents an M×1 column vector, an i^(th) element inl_(i) is 1, remaining elements are 0, and M=L₁×L₂.

A function of l_(i) in W₃ is to indicate a preceding vector, and afunction of ϕ_(n) is to perform phase weighting on two groups ofpolarization antennas. Because first N_(t)/2 rows in the precedingmatrix W correspond to a precoding weighting of an antenna port in onepolarization direction, last N_(t)/2 rows correspond to a precodingweighting of an antenna port in another polarization direction.

In this embodiment of the present invention, in an optionalimplementation, a frequency domain resource corresponding to the firstprecoding matrix indicator is a downlink system bandwidth of theterminal device.

The downlink system bandwidth includes A first sub-bands and B secondsub-bands, A and B are positive integers greater than 1, and A is lessthan or equal to B.

A frequency domain resource corresponding to the second precoding matrixindicator is one of the A first sub-bands, and a frequency domainresource corresponding to the third precoding matrix indicator is one ofthe B second sub-bands.

The downlink system bandwidth may be a downlink system bandwidth of acarrier. For example, if there is only one downlink carrier, thedownlink system bandwidth is a downlink system bandwidth of the carrier.In a carrier aggregation scenario, if there are a plurality of downlinkcarriers, the downlink system bandwidth is a downlink system bandwidthof a carrier corresponding to a CSI fed back by the terminal device. Forexample, there are two downlink carriers: a carrier 1 and a carrier 2.If the terminal device currently feeds back a CSI of the carrier i, thedownlink system bandwidth is a downlink system bandwidth of the carrier1.

Optionally, the terminal device reports a second preceding matrixindicator for each first sub-band, and reports a third precoding matrixindicator for each second sub-band.

In this embodiment of the present invention, in an optionalimplementation, a frequency domain resource of at least one of the Afirst sub-bands is the same as frequency domain resources of at leasttwo of the B second sub-bands.

For example, the entire system bandwidth is divided into A firstsub-bands, and each first sub-band includes S second sub-bands. Theterminal device sends a second precoding matrix indicator to the basestation for the entire system bandwidth. The terminal device sends afirst precoding indicator for the system bandwidth, sends a secondprecoding matrix indicator for each first sub-band, and sends a thirdpreceding matrix indicator for each second sub-band.

The terminal device needs to select a second precoding matrix indicatorfor each first sub-band. The second preceding matrix indicator is usedto select 2K column vectors from the first precoding matrix for thesub-band. In FIG. 5, each first sub-band has four second sub-bands.

For a rank (or rank indication), each precoding matrix W in acorresponding precoding matrix set may be represented as W=W₁×W₂×W₃. Itis assumed that W₁ includes 16 beam vectors.

${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$

and X₁ has 16 columns, which correspond to the 16 beam vectors. Thefollowing gives an example of different antenna port configurations anddifferent W₁ and W₂.

For example, when an antenna port is configured as (N₁, N₂)(8, 2), whereN₁ is a quantity of antenna ports in a horizontal direction, and N₂ is aquantity of antenna ports in a vertical direction, a diagonal matrix ofW₁ is a Kronecker product of eight vectors in the horizontal directionand two vectors in the vertical direction, and there are a total of 16beam vectors. When an antenna port is configured as (N₁, N₂)=(4, 4), abeam vector in W₁ is a Kronecker product of four vectors in thehorizontal direction and four vectors in the vertical direction, andthere also are a total of 16 beam vectors. The vectors in W₁ are dividedinto several vector groups, and each vector group includes four or eightbeam vectors.

Using (N₁, N₂)=(8, 2) as an example, W₁ includes 16 beam vectors. The 16beam vectors are numbered as 1, 2, . . . , and 16. Using eight beams asone beam vector group, and there are four overlapped beam vectorsbetween every two neighboring beam vector groups. Therefore, the 16 beamvectors may be divided into three beam vector groups. The first beamvector group includes beam vectors numbered 1 to 8, the second beamvector group includes beam vectors numbered 5 to 12, and the third beamvector group includes beam vectors numbered 9 to 16. The terminal deviceindicates a selected beam vector group by the second precoding matrixindicator. This is equivalent to that W₂ is determined. In this way, forthe third precoding indicator, three bits may be used to select a beamvector for precoding.

This may alternatively be that W₁ includes 16 beam vectors, which aredivided into four groups, and each group includes four beam vectors. Thefirst beam vector group includes beam vectors numbered 1 to 4, thesecond beam vector group includes beam vectors numbered 5 to 8, thethird beam vector group includes beam vectors numbered 9 to 12, and thefourth beam vector group includes beam vectors numbered 13 to 16. Thereis no overlapped beam vector between neighboring groups. The terminaldevice indicates a selected beam vector group by the second precodingmatrix indicator.

In this embodiment of the present invention, in an optionalimplementation, frequency domain resources corresponding to the firstprecoding matrix indicator, the second precoding matrix indicator, andthe third precoding matrix indicator are downlink system bandwidths ofthe terminal device. This implementation corresponds to widebandfeedback of a precoding matrix indicator. The wideband feedback of theprecoding matrix indicator means that a frequency domain resourcecorresponding to a feedback precoding matrix indicator is an entiresystem bandwidth.

In this embodiment of the present invention, in an optionalimplementation, a transmission period of the first precoding matrixindicator is P₁, a transmission period of the second precoding matrixindicator is P₂, a transmission period of the third precoding matrixindicator is P₃, P₁ is greater than or equal to P₂, and P₂ is greaterthan or equal to P₃.

In this embodiment of the present invention, in an optionalimplementation, the transmission periods P₁, P₂, and P₃ are sent by thebase station to the terminal device through RRC signaling.

Different transmission periods may be configured for different precodingmatrix indicators, and are used to correspond to different features of achannel. Some precoding matrix indicators correspond to a part that isof the channel and that varies relatively fast over time, and someprecoding matrix indicators correspond to a part that is of the channeland that varies relatively slowly over time. For example, the firstprecoding matrix indicator corresponds to a part that is of a channeland that varies most slowly over time, the second precoding matrixindicator corresponds to a part that is of the channel and that variesrelatively slowly over time, and the third precoding matrix indicatorcorresponds to a part that is of the channel and that varies relativelyfast over time. P₁, P₂, and P₃ are configured based on channel features,so as to reduce a quantity of bits for feeding back a PMI.

For example, in an LTE system, a length of one subframe is 1millisecond, which is also a length of one transmission time interval(TTI). The TTI refers to a transmission length for independent decodingon a radio link.

The terminal device feeds back the precoding matrix indicator on aPUCCH. In a first feedback mode, a channel quality indication (CQI) andthe third precoding matrix indicator are reported on the PUCCH in onesubframe. The rank indication and the first precoding matrix indicatorare reported on the PUCCH in one subframe.

A reporting period of the rank indication and the first precoding matrixindicator is P₁, a reporting period of the second preceoding matrixindicator is P₂, and a reporting period of the channel qualityindication and the third precoding matrix indicator is P₃, whereP₁=T₁P₂, and P₂=T₂P₃. For example, T₁=10, and T₂=4.

In a second feedback mode of the PUCCH, the RI, a preceoding typeindication (PTI), and the first precoding matrix indicator are reportedtogether, and there also are three reporting periods: P₁, P₂, and P₃.

At a reporting moment in the period P₁, the terminal device reports theRI, the PTI, and the first precoding matrix indicator on the PUCCH.

At a reporting moment in the period P₂, when PTI=0, the second precodingmatrix indicator is reported. When PTI=1, a wideband CQI and wideband W₃are reported. Wideband CQI reporting (wideband CQI feedback) means thata frequency domain resource corresponding to a feedback CQI is an entiresystem bandwidth.

At a reporting moment in the period P₃, when PTI=0, a wideband CQI andwideband W₃ are reported. When PTI=1, a third precoding matrix indicatorcorresponding to a sub-band CQI and sub-band W₃ are reported, whereP₁=T₁P₂, and P₂=T₂P₃. For example, T₁=8, and T₂=5.

In this embodiment of the present invention, in an optionalimplementation, T column vectors in 2K column vectors in W₂ areindicated by the first precoding matrix indicator, T is an integergreater than or equal to 2, and T is less than K. 2K-T column vectors inW₂ except the T column vectors are indicated by the T column vectors andthe second precoding matrix indicator.

For example, based on the first precoding matrix indicator, 16 beamvectors are determined as [v₁ . . . v₁₆] where [v₁ v₂ v₃ v₄] aremandatory beam vectors. The second precoding matrix indicator is notrequired, and the second precoding matrix indicator is only used toindicate another vector group. For example, 1 is used to indicate arelationship between a second vector group and a first vector group. Forexample, W₁×W₂ may also be represented as

${{W_{1}W_{2}} = \begin{bmatrix}Y & 0 \\0 & Y\end{bmatrix}},$

where Y=[v₁ v₂ v₃ v₄ v_(1+i) . . . v_(4+l)], and Y is a matrix ofN_(t)/2 rows and K columns. The second vector group may alternatively bedirectly indicated. For example, the first precoding matrix indicatorcorresponds to four vector groups, where the first vector group is amandatory vector group. The second precoding matrix indicator indicatesan another selected vector group. Compared with selecting a plurality ofgroups of vectors from W₁ by the second precoding matrix indicator, inthe present invention, a vector group that needs to be indicated isreduced by using the second preceding matrix indicator, thereby reducinga quantity of bits in feedback. For example, the second precoding matrixchanges from indicating two vector groups to indicating one vectorgroup, reducing a quantity of bits required for indicating one vectorgroup.

In this embodiment of the present invention, in an optionalimplementation, 2K column vectors in W₂ are indicated by a configurationparameter delivered by a base station and the second preceding matrixindicator.

For example, W₁ includes 16 beam vectors, which are divided into fourgroups, and each group includes four vectors. The first vector groupincludes beam vectors numbered 1 to 4, the second vector group includesbeam vectors numbered 5 to 8, the third vector group includes beamvectors numbered 9 to 12, and the fourth vector group includes beamvectors numbered 13 to 16. As shown in this example, the terminal devicesends indicators of the first vector group and the fourth vector groupto the base station by using the second preceding matrix indicator, andthere are a total of eight column vectors. To further reduce a quantityof bits of a preceding indicator sent by the terminal device, theselected vector group may further be downsampled based on theconfiguration parameter. For example, when the configuration parameteris 1, first to fourth vectors are selected from the eighth vectors. Whenthe configuration parameter is 2, the first, second, fifth, and sixthcolumn vectors are selected from the eighth vectors. When theconfiguration parameter is 3, the first, third, sixth, and eighth columnvectors are selected from the eighth vectors. When the configurationparameter is 4, the first, fourth, fifth, and eighth column vectors areselected from the eighth vectors. For example, the terminal deviceindicates eight vectors, which are denoted as v₁, v₂, . . . , and v₈ bythe second precoding matrix indicator. If no configuration parameter isused to downsample the selected vector groups,

${{W_{1} \times W_{2}} = \begin{bmatrix}Y & 0 \\0 & Y\end{bmatrix}},$

Y=[v₁ v₂ . . . v₈], including eight vectors. Further, W₁×W₂ includeseight beam vectors. If there is a configuration parameter used todownsample the selected vector groups, and the configuration parameteris 1,

${{W_{1} \times W_{2}} = \begin{bmatrix}Y & 0 \\0 & Y\end{bmatrix}},$

Y=[v₁ v₂ v₃ v₄], including four vectors. Further, W₁×W₂ includes fourbeam vectors. Because the configuration parameter is configured by thebase station, the configuration parameter is fixed after configuration,and therefore does not occupy a feedback bit of the terminal device. Inthis way, for the third precoding indicator, only two bits may be usedto select a beam vector for precoding. If there is no configurationparameter, 3 bits need to be used to select a beam vector for precoding.Therefore, in this implementation, a quantity of bits for feeding backthe third preceding indicator can be reduced.

Optionally, the configuration parameter may be sent by the base stationto the terminal device through RRC signaling.

In this embodiment of the present invention, in an optionalimplementation, the configuration parameter is used to indicate aselectable column vector set of W₁, the selectable column vector setincludes J column vectors, and J satisfies 2K<J<2M.

For example, the first precoding indicator indicates that W₁ includes 16beam vectors. Using four beams as one beam vector group, there may befour beam groups after division. Two of the beam vector groups aredetermined by using the configuration parameter. For example, the firstand the third beam vector groups are determined by using theconfiguration parameter. Because each vector group has four beamvectors, a total of eight beam vectors are selected by using theconfiguration parameter. This is equivalent to that in thisimplementation, J=8×2=16. It is further determined, based on the secondprecoding indicator, whether to select the first beam vector group orthe third beam vector group for subsequent processing. A quantity ofbits for feeding back the second precoding indicator is reduced by usingthe configuration parameter.

In this embodiment of the present invention, in an optionalimplementation, X₁ in W₁ satisfies X₁=[v₀ ¹ . . . v_(M) ₁ ₋₁ ¹]⊗[v₀ ² .. . v_(M) ₂ ₋₁ ²] where

v_(m) ¹ is a column vector including N₁ elements, v_(n) ² is a columnvector including N₂ elements, N₁×N₂=N_(t)/2, M₁×M₂=M, and ⊗ represents aKronecker product.

In this embodiment of the present invention, in an optionalimplementation, X₂ in W₂ satisfies X₂=X₃ ⊗X₄, where X₃ is a matrix of M₁rows and K₁ columns, X₄ is a matrix of M₂ rows and K₂ columns, and ⊗represents a Kronecker product.

Any column in X₃ is represented as l_(i), l_(i) is an M₁×1 columnvector, an i^(th) element in l_(i) is 1, remaining elements are 0, avalue of is an integer from 1 to M₁.

Any column in X₄ is represented as l_(j), l_(j) is an M₂×1 columnvector, a j^(th) element in l_(j) is 1, remaining elements are 0, avalue of j is an integer from 1 to M₂.

K ₁ ×K ₂ =K, and M ₁ ×M ₂ =M.

In the foregoing embodiment, there may be no preference in order betweenstep 102 and step 103, and determining in the two steps may be performedsimultaneously. This is because when the precoding matrix that theterminal device expects the base station to use is determined, thecorresponding precoding matrix indicator is determined.

Step 101 may be performed before steps 102 and 103. Alternatively, step101 and steps 102 and 103 may be performed simultaneously.

For example, a received signal model of the UE is:

y=HWs+n  (2)

where y is a received signal vector, H is a channel matrix, w is aprecoding matrix, _(s) is a transmitted symbol vector, and _(n) isinterference plus noise.

The terminal device traverses all ranks and all precoding matricescorresponding to the ranks, and calculates a channel capacity obtainedafter precoding is performed on each precoding matrix. A channelcapacity is obtained for each precoding matrix. The channel capacity maybe a quantity of bits that can be correctly sent at the transmittingend. A precoding matrix corresponding to a maximum channel capacity anda rank corresponding to the precoding matrix are obtained. The terminaldevice sends, to the base station, a rank indication corresponding tothe precoding matrix and a precoding matrix indicator corresponding tothe precoding matrix.

When a rank is determined, for example, in a subframe, the terminaldevice needs to send a precoding matrix indicator. A rank indication hasbeen sent before. In this case, the terminal device only needs totraverse a precoding matrix set corresponding to the rank indication.For example, a rank corresponding to the rank indication is 1, theterminal device needs to traverse only a precoding matrix set for whicha rank is equal to 1, so as to obtain a precoding matrix allowing amaximum channel capacity; and sends a corresponding precoding matrixindicator to the base station.

When traversing a precoding matrix corresponding to a rank, the terminaldevice may obtain the precoding matrix by a precoding matrix indicator.For example, during precoding matrix indicator traversing, when aprecoding matrix indicator is traversed, a precoding matrix is obtainedbased on the precoding matrix indicator, and a channel capacity iscalculated based on the precoding matrix. Alternatively, a precodingmatrix may be directly traversed. After a precoding matrix allowing amaximum channel capacity is selected, a precoding matrix indicator isobtained based on a correspondence between a precoding matrix and aprecoding matrix indicator. The precoding matrix indicator is sent tothe base station.

The method according to this embodiment of the present invention isdescribed in detail in the foregoing with reference to FIG. 3. Aterminal device and a base station according to embodiments of thepresent invention are described below with reference to FIG. 6 to FIG.9.

FIG. 6 is a schematic block diagram of a terminal device according to anembodiment of the present invention. As shown in FIG. 6, the terminaldevice 600 includes a processing unit 601. The processing unit 601 isconfigured to: determine a rank indication, and determine a precodingmatrix W in a precoding matrix set corresponding to the rank indication.W satisfies W=W₁×W₂×W₃, W is a matrix of N_(t) rows and R columns, N_(t)is a quantity of antenna ports, R is a rank value corresponding to therank indication, N_(t) is greater than or equal to R. W₁ is a matrix ofN_(t) rows and 2M columns, W₂ is a matrix of 2M rows and 2K columns, W₃is a matrix of 2K rows and R columns. K is less than M, N_(t), R, M, andK are all positive integers, M is greater than or equal to 2, N_(t) isan even number. None of W₁, W₂, and W₃ is an identity matrix, and the 2Mcolumns in W₁ include every column in W₁×W₂.

The precoding matrix W corresponds to a first precoding matrixindicator, a second precoding matrix indicator, and a third precodingmatrix indicator, the first precoding matrix indicator corresponds tothe first precoding matrix W₁, the second precoding matrix indicatorcorresponds to the second precoding matrix W₂, and the third precodingmatrix indicator corresponds to the third precoding matrix W₃.

The terminal device 600 includes a sending unit 602 which is configuredto send the rank indication, the first precoding matrix indicator, thesecond precoding matrix indicator, and the third precoding matrixindicator.

The terminal device further includes a receiving unit 603, configured toreceive a configuration parameter sent by a base station.

For further descriptions of the rank indication, the first precodingmatrix indicator, the second precoding matrix indicator, the thirdprecoding matrix indicator, and the preceding matrix W, refer todescriptions in the method embodiment of the present invention. Forspecific implementation of the processing unit of the terminal device,refer to specific implementation of the terminal device in the foregoingmethod embodiment.

Therefore, based on the terminal device sending the precoding matrixindicator in this embodiment of the present invention, a quantity ofbits for feeding back the precoding matrix indicator is reduced while asystem performance requirement is satisfied.

FIG. 7 is a schematic block diagram of a base station according to anembodiment of the present invention.

The base station 700 includes a receiving unit 701. The receiving unit701 is configured to receive a rank indication, a first precoding matrixindicator, a second precoding matrix indicator, and a third precodingmatrix indicator that are sent by a terminal device.

The base station 700 includes a processing unit 702 which is configuredto determine, in a precoding matrix set corresponding to the rankindication, a precoding matrix W based on the first precoding matrixindicator, the second precoding matrix indicator, and the thirdprecoding matrix indicator.

W satisfies W=W₁×W₂×W₃, W is a matrix of N_(t) rows and R columns, N_(t)is a quantity of antenna ports, R is a rank value corresponding to therank indication, N_(t) is greater than or equal to R, W₁ is a matrix ofN_(t) rows and 2M columns, W₂ is a matrix of 2M rows and 2K columns, W₃is a matrix of 2K rows and R columns, K is less than M, N_(t), R, M, andK are all positive integers, M is greater than or equal to 2, N_(t) isan even number, none of W₁, W₂, and W₃ is an identity matrix, and the 2Mcolumns in W₁ include every column in W₁×W₂.

The first precoding matrix indicator corresponds to the first precodingmatrix W₁, the second precoding matrix indicator corresponds to thesecond precoding matrix W₂, and the third precoding matrix indicatorcorresponds to the third precoding matrix W₃.

The base station further includes a sending unit 703, configured to senda configuration parameter.

For further descriptions of the rank indication, the first precodingmatrix indicator, the second precoding matrix indicator, the thirdprecoding matrix indicator, and the precoding matrix W, refer todescriptions in the method embodiment of the present invention. Forspecific implementation of the processing unit of the base station,refer to specific implementation of the base station in the foregoingmethod embodiment.

Therefore, based on the base station receiving the precoding matrixindicator in this embodiment of the present invention, a quantity ofbits of the received precoding matrix indicator is reduced while asystem performance requirement is satisfied.

The processing unit may be a processor, the receiving unit may be areceiver, and sending unit may be a transmitter. A terminal deviceincluding a processor 801, a transmitter 802, and a receiver 803 isshown in FIG. 8. A base station including a processor 902, a transmitter903, and a receiver 901 is shown in FIG. 9.

It should be understood that in the embodiments of the presentinvention, the processor 801/901 may be a central processing unit(Central Processing Unit, “CPU” for short), or the processor 801/901 maybe another general purpose processor, a digital signal processor (DSP),an application-specific integrated circuit (ASIC), a field programmablegate array (FPGA) or another programmable logical device, a discretegate or a transistor logical device, a discrete hardware component, orthe like. The general purpose processor may be a microprocessor, or theprocessor may be any conventional processor or the like.

The objectives, technical solutions, and benefits of the presentinvention are further described in detail in the foregoing specificembodiments. It should be understood that the foregoing descriptions aremerely specific embodiments of the present invention, but are notintended to limit the protection scope of the present invention. Anymodification, equivalent replacement, or improvement made within thespirit and principle of the present invention shall fall within theprotection scope of the present invention.

1. A method for wireless communications, comprising: receiving, by anetwork device, a rank indication, a first precoding matrix indicator, asecond precoding matrix indicator, and a third precoding matrixindicator from a terminal device; and determining, by the networkdevice, a precoding matrix W based on the rank indication, the firstprecoding matrix indicator, the second precoding matrix indicator, andthe third precoding matrix indicator, wherein W satisfies W=W₁×W₂×W₃, Wis a matrix of N_(t) rows and R columns, N_(t) is a quantity of antennaports, R is a rank value corresponding to the rank indication, N_(t) isgreater than or equal to R; W₁ is a matrix of N_(t) rows and 2M columns,W₂ is a matrix of 2M rows and 2K columns, W₃ is a matrix of 2K rows andR columns, K is less than M; N_(t), R, M, and K are all positiveintegers, M is greater than or equal to 2, N_(t) is an even number, noneof W₁, W₂, and W₃ is an identity matrix, and the 2M columns in W₁comprise every column in W₁×W₂; wherein the first precoding matrixindicator corresponds to the first precoding matrix W₁, the secondprecoding matrix indicator corresponds to the second precoding matrixW₂, and the third precoding matrix indicator corresponds to the thirdprecoding matrix W₃.
 2. The method according to claim 1, wherein: W₂satisfies ${W_{2} = \begin{bmatrix}X_{2} & 0 \\0 & X_{2}\end{bmatrix}},$ X₂ is a matrix of M rows and K columns, any column inX₂ is represented as e_(p), e_(p) is an M×1 column vector; a p^(th)element in $W_{2} = \begin{bmatrix}X_{2} & 0 \\0 & X_{2}\end{bmatrix}$ is 1, and the remaining elements are 0, wherein p is aninteger from 1 to M.
 3. The method according to claim 1, wherein: W₁satisfies ${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$ X₁ is a matrix of N_(t)/2 rows and M columns, X₁=[v₀ .. . v_(M-1)], and each element in X₁ is a column vector comprisingN_(t)/2 elements.
 4. The method according to claim 1, wherein: afrequency domain resource corresponding to the first precoding matrixindicator is a downlink system bandwidth of the terminal device, thedownlink system bandwidth comprises A first sub-bands and B secondsub-bands, A and B are positive integers greater than 1, and A is lessthan or equal to B; and a frequency domain resource corresponding to thesecond precoding matrix indicator is one of the A first sub-bands, and afrequency domain resource corresponding to the third precoding matrixindicator is one of the B second sub-bands.
 5. The method according toclaim 1, wherein a transmission period of the first precoding matrixindicator is P₁, a transmission period of the second precoding matrixindicator is P₂, a transmission period of the third precoding matrixindicator is P₃, P₁ is greater than or equal to P₂, and P₂ is greaterthan or equal to P₃.
 6. The method according to claim 1, wherein Tcolumn vectors in 2K column vectors in W₂ are indicated by the firstprecoding matrix indicator, the remaining 2K-T column vectors in W₂ areindicated by the T column vectors and the second precoding matrixindicator, T is an integer greater than or equal to 2, and T is lessthan K.
 7. The method according to claim 1, wherein 2K column vectors inW₂ are indicated by a configuration parameter and the second precodingmatrix indicator.
 8. A communication device, comprising: a receiver, thereceiver configured to receive a rank indication, a first precodingmatrix indicator, a second precoding matrix indicator, and a thirdprecoding matrix indicator from a terminal device; and a processor, theprocessor configured to determine a precoding matrix W based on the rankindication, the first precoding matrix indicator, the second precodingmatrix indicator, and the third precoding matrix indicator, wherein Wsatisfies W=W₁×W₂×W₃, W is a matrix of N_(t) rows and R columns, N_(t)is a quantity of antenna ports, R is a rank value corresponding to therank indication, N_(t) is greater than or equal to R; W₁ is a matrix ofN_(t) rows and 2M columns, W₂ is a matrix of 2M rows and 2K columns, W₃is a matrix of 2K rows and R columns, K is less than M; N_(t), R, M, andK are all positive integers, M is greater than or equal to 2, N_(t) isan even number, none of W₁, W₂, and W₃ is an identity matrix, and the 2Mcolumns in W₁ comprise every column in W₁×W₂; wherein the firstprecoding matrix indicator corresponds to the first precoding matrix W₁,the second precoding matrix indicator corresponds to the secondprecoding matrix W₂, and the third precoding matrix indicatorcorresponds to the third precoding matrix W₃.
 9. The communicationdevice according to claim 8, wherein: W₂ satisfies${W_{2} = \begin{bmatrix}X_{2} & 0 \\0 & X_{2}\end{bmatrix}},$ X₂ is a matrix of M rows and K columns, any column inX₂ is represented as e_(p), e_(p) is an M×1 column vector; a p^(th)element in $W_{2} = \begin{bmatrix}X_{2} & 0 \\0 & X_{2}\end{bmatrix}$ is 1, and the remaining elements are 0, wherein p is aninteger from 1 to M.
 10. The communication device according to claim 8,wherein: W₁ satisfies ${W_{1} = \begin{bmatrix}X_{1} & 0 \\0 & X_{1}\end{bmatrix}},$ X₁ is a matrix of N_(t)/2 rows and M columns, X₁=[v₀ .. . v_(M-1)], and each element in X₁ is a column vector comprisingN_(t)/2 elements.
 11. The communication device according to claim 8,wherein: a frequency domain resource corresponding to the firstprecoding matrix indicator is a downlink system bandwidth of theterminal device, the downlink system bandwidth comprises A firstsub-bands and B second sub-bands, A and B are positive integers greaterthan 1, and A is less than or equal to B; and a frequency domainresource corresponding to the second precoding matrix indicator is oneof the A first sub-bands, and a frequency domain resource correspondingto the third precoding matrix indicator is one of the B secondsub-bands.
 12. The communication device according to claim 8, wherein atransmission period of the first precoding matrix indicator is P₁, atransmission period of the second precoding matrix indicator is P₂, atransmission period of the third precoding matrix indicator is P₃,wherein P₁ is greater than or equal to P₂, and P₂ is greater than orequal to P₃.
 13. The communication device according to claim 8, whereinT column vectors in 2K column vectors in W₂ are indicated by the firstprecoding matrix indicator, the remaining 2K-T column vectors in W₂ areindicated by the T column vectors and the second precoding matrixindicator, T is an integer greater than or equal to 2, and T is lessthan K.
 14. The communication device according to claim 8, furthercomprising a transmitter, wherein the transmitter is configured to senda configuration parameter, and wherein 2K column vectors in W₂ areindicated by the configuration parameter and the second precoding matrixindicator. 15-16. (canceled)
 17. The communication device of claim 8,wherein the processor includes at least one of a central processing unit(CPU), a digital signal processor (DSP), an application-specificintegrated circuit (ASIC) or a field programmable gate array (FPGA). 18.The communication device of claim 17, wherein the communication deviceis a base station.